Carbon dioxide (CO2) is an environmentally friendly, naturally abundant, non-polar molecule. Being non-polar, CO2 has the capacity to dissolve a variety of non-polar materials or contaminates. The degree to which the contaminants found in non-polar CO2 are soluble is dependant on the physical state of the CO2. The four phases of CO2 are solid, liquid, gas, and supercritical. The four phases or states are differentiated by appropriate combinations of specific pressures and temperatures. CO2 in a supercritical state (sc-CO2) is neither liquid nor gas but embodies properties of both. In addition, sc-CO2 lacks any meaningful surface tension while interacting with solid surfaces, and hence, can readily penetrate high aspect ratio geometrical features more readily than liquid CO2. Moreover, because of its low viscosity and liquid-like characteristics, the sc-CO2 can easily dissolve large quantities of many other chemicals. It has been shown that as the temperature and pressure are increased into the supercritical phase, the solubility of CO2 also increases. This increase in solubility has lead to the development of a number of sc-CO2 cleaning processes.
One problem in semiconductor manufacturing is that the cleaning process sometimes does not completely remove photoresist residue and other residues and contaminants on the surface of the wafer. For example, during some cleaning processes, oxidized residues can be created that adversely affect the cleaning process. It would be advantageous during the cleaning process to be able to remove both oxidized and non-oxidized residues and/or contaminants from the surface features on the wafer surface.
What is needed is an improved method for removing oxidized and/or partially oxidized residues from substrate surfaces.